Bipolar junction transistors (BJTs) are key parts of analog integrated circuits. BJTs can be grouped as NPN BJTs (NBJTs) and PNP BJTs (PBJTs). A symbol for an NPN BJT, which comprises collector C, base B, and emitter E, is shown in FIG. 1A. There are two types of commonly designed BJTs that can be formed using conventional CMOS technology. FIGS. 1B and 1C illustrate a vertical-BJT, wherein a top view is shown in FIG. 1B and a cross-sectional view is shown in FIG. 1C. The vertical-BJT is an NPN transistor comprising doped regions, namely emitter E, base contact B and collector C. To enhance emitter injection efficiency, emitter E is encircled by base contact B and collector C. Emitter E and base contact B are formed in a P-well and collector C is formed in an N-well, wherein the P-well and the N-well are further formed on a deep N-well (DNW). Since the base regions include the P-well and base contact B, the emitter/base junction and the collector/base junction are formed between vertically located components, and the respective BJT is referred to as a vertical BJT.
FIGS. 1D and 1E illustrate an (NPN) lateral-BJT. A top view is shown in FIG. 1D and a cross-sectional view is shown in FIG. 1E. Since the P-well, which is part of the base region, has a portion inserted between emitter E and collector C, the emitter/base junction and the collector/base junction are formed between laterally located components, and the respective BJT is referred to as a lateral-BJT (LBJT).
The ability for improving the gain of conventional vertical-BJTs and lateral-BJTs is limited for the following reasons. Using FIG. 1E as an example, besides the intentionally formed BJT, there is also a parasitic transistor NBJT, whose collector, base and emitter are formed by the region E, the P-well and the deep N-well DNW, respectively. Since a significant portion of the emitter/base junction of the parasitic transistor NBJT is located at interface 2, which is located at the bottom of emitter E, and reduces the effect of the parasitic transistor NBJT, it is preferred for interface 2 to have a small area. On the other hand, in order to improve the emitter injection efficiency of the lateral-BJT, it is preferred for the path between emitter E and collector C, which is symbolized by arrows 4, to have a big interface area (in the plane perpendicular to the illustrated plane). This requires the length L (FIG. 1D) of emitter E and collector C to have a great value. A great length L and a small area for emitter E are conflicting requirements, which means that the improvement of the lateral-BJT comes with the cost of a greater parasitic BJT. A dilemma for conventional vertical-BJTs is that the base width, which is substantially equal to a depth of the P-well as shown in FIG. 1C, is not scalable, if the vertical-BJTs are formed using a same technology as the formation of CMOS devices.